[1] S. labiatus has a laterally compressed, knife-shaped body with a nearly straight dorsal profile and a long anal fin.

The head is laterally compressed, with a curved dorsal profile and small eyes that are covered by a thin membrane.

The mouth is terminal, with a three-lobed rigid structure consisting of a pair of smooth, hard, bulbous swellings along the lower lip and a medial spur that projects down and forward.

The lower jaw is longer than deep, bearing two rows of conical teeth numbering 17 plus 4 replacements each.

Like other apteronotids, these fish generate a continuous weak electric field for the purposes of electrolocation and communication.